Currently, automobile parts such as gears, transmissions and seat recliners are manufactured by cold working hot rolled steel sheets, which are carbon steel for machine structural use specified in JIS G 4051, into desired shapes and thereafter hardening the steel by quenching to ensure desired hardness. Thus, excellent cold workability (formability) and hardenability are required for hot rolled steel sheets that are materials. Various such steel sheets have been proposed.
For example, Patent Literature 1 discloses a hot rolled steel sheet with excellent toughness after hardening which includes, by mass %, C: 0.10 to 0.37%, Si: not more than 1%, Mn: not more than 1.4%, P: not more than 0.1%, S: not more than 0.03%, sol. Al: 0.01 to 0.1%, N: 0.0005 to 0.0050%, Ti: 0.005 to 0.05% and B: 0.0003 to 0.0050% wherein B—(10.8/14)N*≥0.0005% and N*=N−(14/48)Ti with the proviso that N*=0 when the right side ≤0, the balance being Fe and inevitable impurities, and which has an average particle size of TiN precipitate in the steel of 0.06 to 0.30 μm and a grain size of prior austenite after hardening of 2 to 25 μm.
Patent Literature 2 discloses a method for manufacturing Ti—B high-carbon steel sheets exhibiting excellent cold workability, hardenability and toughness after heat treatment, the steel sheets including, by mass %, C: 0.15 to 0.40%, Si: not more than 0.35%, Mn: 0.6 to 1.50%, P: not more than 0.030%, S: not more than 0.020%, sol. Al: 0.01 to 0.20%, N: 0.0020 to 0.012%, Ti: 0.005 to 0.1% and B: 0.0003 to 0.0030% wherein B≤0.0032 - 0.014×sol. Al−0.029×Ti, the balance being Fe and inevitable impurities.
Patent Literature 3 discloses a high-carbon hot rolled steel sheet with excellent cold workability which has a chemical composition including, by mass %, C: 0.20 to 0.48%, Si: not more than 0.1%, Mn: 0.20 to 0.60%, P: not more than 0.02%, S: not more than 0.01%, sol. Al: not more than 0.1%, N: not more than 0.005%, Ti: 0.005 to 0.05%, B: 0.0005 to 0.003% and Cr: 0.05 to 0.3% wherein Ti - (48/14)N 0.005, the balance being Fe and inevitable impurities, and which includes a microstructure having an average ferrite grain size of not more than 6 μm, an average carbide particle size of from 0.1 μm to less than 1.20 μm, and a volume fraction of substantially carbide-free ferrite grains of not more than 5%.